In the Internet, hosts send packets of data.

• Hosts send data:
  • Application data is split into smaller chunks (i.e. packets which encapsulate application message) of length $L$ bits
  • Packets are transmitted into access network at transmission rate $R$
  • Link transmission rate - speed at which we can push bits to link - $\frac L R$ seconds to push data
• Network moves packets of data:
  • Done via packet-switching - network forwards packets from one router to the next, across links on path from source to destination

There are 2 main functions of network-cores (i.e. interconnected routers):

• Forwarding (switching) - local action
  • Move arriving packets from router’s input link to appropriate router output link
  • Use local forwarding table to determining output link for a given header value
• Routing - global action
  • Determines source-destination paths taken by packets
  • Routing algorithm picks particular output link to send to
    • Note there are different metrics you could design a routing algorithm around

A router will implement both forwarding and routing.

Packet-Switching: Store-and-forward

Packet switches move packets from some source to a destination.

• Each packet is completely independent - pieces are encapsulated in its own packet + switched independently
• Packet transmission delay: it takes $L/R$ seconds to transmit an $L$-bit packet into a link at $R$ bits/second
• Store & Forward ⇒ the entire packet must arrive at a router before it can be transmitted to the next link
  • We can’t just look at the header, as more processing may be required
    • e.g. bit flip errors, etc. - don’t forward packet if errors

As an entire packet must arrive at a router before it can be forwarded, a transmission delay occurs (due to wait time).

• Given $p$ routers, total delay time is $(p+1)L/R$ ($p$ routers ⇒ $p+1$ links) - single packet
• For $n$ packets and transmission rate of $R$ bits/second, delay is $pL/R + nL/R$ (assuming same delay between routers)

Packet-Switching: Queueing

Queueing occurs when packets arrive faster than they can be processed/serviced.